The present invention relates generally to a method for forming high-alumina bodies, and, more particularly, to a method for sintering high alumina bodies having superior properties and at reduced temperatures.
Alumina (also known as Al2O3 or corundum) is a useful and ubiquitous ceramic material. Alumina is a very hard crystalline material. It has a structure that may be described as a hexagonal close-pack array of oxygen atoms with metal atoms in two-thirds of the octahedrally coordinated interstices. Each metal atom is thus coordinated by six oxygen atoms, each of which has four metal neighbors (6:4 coordination). Alumina products include abrasives, insulators, structural members, refractory bricks, electronic substrates, and tools. Alumina is stable, hard, lightweight, and wear resistant, making it attractive for such applications as seal rings, air bearings, electrical insulators, valves, thread guides, and the like, as well as the ceramic reinforcing component in metal matrix composites.
Alumina is produced on an industrial scale using the Bayer Process to separate ferric oxide, silica and aluminum oxides. Bauxite ore is ground finely then treated with sodium hydroxide (NaOH) in an iron autoclave at an elevated temperature. The alumina dissolves as sodium aluminate via the equation:
Al2O3+2NaOHxe2x86x922NaAlO2+H2O
The silica dissolves to form sodium silicate but the ferric oxide, being insoluble, is filtered off. Carbon dioxide is then passed through the solution, decomposing the sodium aluminate (NaAlO2) to form aluminum hydroxide and sodium carbonate:
2NaAlO2+CO2xe2x86x92Na2CO3+↓2Al(OH)3
The aluminum hydroxide is separated by filtration and calcined at 1000xc2x0 C. or higher, when it loses its water of constitution, yielding alumina:
2Al(OH)3xe2x86x92Al2O3+3H2O
Pure crystalline alumina is a very inert substance and resists most aqueous acids and alkalis. It is more practical to use either alkaline (NaOH) or acidic (KHSO4, KHF2, etc) melts. Concentrated boiling sulfuric acid also can be used as an etchant.
In order to produce useful bodies, alumina must be densified or sintered. Sintering is the process in which a compact of a crystalline powder is heat treated to form a single coherent solid. The driving force for sintering is the reduction in the free surface energy of the system. This is accomplished by a combination of two processes, the conversion of small particles into fewer larger ones (particle and grain growth) and coarsening, or the replacement of the gas solid interface by a lower energy solid solid interface (densification). This process is modeled in three stages:
Initialxe2x80x94the individual particles are bonded together by the growth of necks between the particles and a grain boundary forms at the junction of the two particles.
Intermediatexe2x80x94characterized by interconnected networks of particles and pores.
Finalxe2x80x94the structure consists of space-filling polyhedra and isolated pores.
The kinetics of sintering tend to be temperature sensitive, such that an increase in sintering temperature generally substantially accelerates the sintering process. In industrial applications, while an increase in sintering temperature decreases sintering time and increases throughput, the economic gains therefrom are offset by increased fuel costs and decreased furnace life (since higher firing temperatures result in more rapid degradation of both the furnace refractory structure and heating elements.) Therefore, an economically optimum sintering temperature is one that best balances gains from throughput with losses from fuel and furnace wear and tear.
The sintering of alumina at temperatures above 1600xc2x0 C. is generally required to achieve a high density, and alumina is commonly sintered in the temperature range of 1700-1800xc2x0 C., since higher temperatures promote more rapid sintering. Sintered alumina bodies reflect the properties of the constituent alumina crystallites or grains, such that they are hard, tough, substantially inert, and resistant to chemical attack (such as dissolution, corrosion and/or degradation from acid or alkaline agents). Mechanical and/or chemical failure of sintered alumina bodies usually occurs as a grain boundary phenomenon. Since the grain boundaries usually contain porosity and a glassy phase, a sintered alumina body is not as hard, tough, inert, and/or chemically resistant as a comparable single crystal alumina body.
One increasingly important use of alumina is as a ceramic phase dispersed in a metal matrix to form a metal matrix composite (MMC). MMCs exhibit properties of both the metal matrix and the dispersed ceramic phase, such that they have the toughness and ductility of the metal matrix combined with the compression strength and vibration dampening characteristics of the dispersed ceramic phase. Since MMCs are true composites, the ceramic phase is merely suspended in the metal matrix and not alloyed therewith. One problem with the production of MMCs is that the metal matrix, if heated to the point of melting, is generally corrosive to the ceramic phase. It therefore is generally necessary to either use more expensive powder metallurgical processing techniques to form MMCs or to very finely control the time during which the molten metal is in contact with the ceramic phase. Contact between the molten metal and the ceramic phase tends to degrade the ceramic phase, resulting in a reduction of the desired ceramic phase and the formation of a region of uncontrolled composition (such as uncontrolled alloying and/or oxide formation) around the remaining (if any) ceramic particles. Degradation of the ceramic phase accordingly results in uncontrolled degradation of the physical and chemical properties of the MMC material, such as a reduction of the toughness, strength and ductility of the MMC. This problem is especially acute if the matrix metal is aluminum, as molten aluminum is very corrosive.
Another problem with MMCs in general and aluminum-based MMCs in particular arises from the difficulty in making a joint in the material without degrading the ceramic phase and weakening the MMC. Typical welding processes form a weld pool of molten metal wherein two pieces may be alloyed together to form a weld joint. In the case of welding an MMC, degradation of the ceramic phase by the molten metal in the weld pool typically produces an undesired contaminant in the weld pool of variable and uncontrolled composition. The presence of the contaminant makes welding difficult, and weakens and embrittles the weld joint material such that catastrophic failure of the part is much more likely to occur at the joint, provided it was possible to form a joint at all. Typical aluminum-based MMCs exhibit degradation of the alumina particles when heated above about 300 degrees Fahrenheit, since solid-state diffusion processes begin to occur between the alumina particles"" grain boundary phases and the aluminum matrix well below the melting point of aluminum. While some more exotic welding techniques, such as high energy x-ray welding have shown promise in the joining of MMCs, they require relatively rare and expensive synchrotronic x-ray sources.
There is therefore a need for a technique for decreasing the sintering temperature of alumina without sacrificing throughput (increasing the sintering time) or quality. There is also a need for producing sintered alumina bodies having bulk physical characteristics closer to those of single-crystal alumina, such as resistance to dissolution in aluminum metal. The present invention addresses these needs.
One form of the present invention relates to substantially non-vitreous high-alumina bodies formed through a low-temperature sintering process. The high-alumina bodies so produced have a substantial resistance to dissolution in molten aluminum and are thus ideal for use as the ceramic oxide component in an aluminum-based metal matrix composite. The effective sintering temperature for a given sintering time required to achieve substantially full densification of the alumina particles was decreased through the addition of quantities of magnesia to the alumina precursor powders. The resulting substantially fully dense high-alumina bodies exhibited superior resistance to chemical attack over a broad range of pH and temperature conditions.
One object of the present invention is to provide a method for producing substantially dense high-alumina bodies at lower sintering temperatures when sintered for comparable times. Related objects and advantages will become apparent from the following description.